This invention relates to a mounting structure of a package, which incorporates at least one of a high-frequency transformer, MIC (Microwave Integrated Circuit) and MMIC (Monolithic Microwave Integrated Circuit) used in the microwave to millimeter-wave band, or of a superconducting circuit board. More particularly, the invention relates to a mounting structure whereby a thermally excellent state can be obtained without sacrificing the microwave characteristic.
Any of the following methods has been implemented heretofore in order to mount a microwave circuit, which handles signals in the microwave to millimeter-wave band, on a case (heat sink): {circle around (1)} the circuit is mounted upon interposing a soft metal such as indium between the case and the package or board; {circle around (2)} the circuit is mounted upon applying a silicon compound or grease between the case and the package or board; {circle around (3)} the circuit is mounted directly without doing any of these; and {circle around (4)} the circuit is mounted on the case directly by soldering.
Further, in the case of a low-frequency circuit, use has been made of the fact that a graphite sheet exhibits thermal anisotropy, wherein the heat that evolves from the heated portion of the low-frequency circuit is transported to a remote location by the graphite sheet so that the heat is dissipated. A further method that has been employed similarly makes use of the properties of a graphite sheet, interposes the graphite sheet between the heat-producing portion of the package and the case and applies pressure to the package as by auxiliary hardware to allow heat to escape. However, these expedients are applied to low-frequency circuits and are not implemented in the case of microwave circuits.
Graphite is an allotrope of carbon and in one sense is akin to diamond but in another is merely the lead of a pencil. A graphite sheet is obtained by molding graphite, which is close to diamond that has an attractive crystal alignment, into sheet form and exhibits a high thermal conductivity second to that of diamond. The thermal conductivity of copper or aluminum is less than one-half that of graphite in one direction. For these reasons, a graphite sheet is ideal as a heat transfer material. However, since a graphite sheet is not in liquid form as in the manner of grease, making it adhere perfectly is a difficult challenge.
Problems of Mounting Structures for Microwave and Millimeter-Wave Circuits
In the usual mounting structure of a microwave and millimeter-wave band circuit, a package internally incorporating a transistor, MIC or MMIC that evolves a high degree of heat is mounted on a metal case and a circuit board on which microstrips have been patterned is disposed about the package. In order to facilitate the dissipation of heat produced by the package, the area of contact between the package and the case has its surface roughness removed to improve planarity and provide as much surface contact as possible.
In the microwave and millimeter-wave band, however, it is required that the lower part of the package be grounded in its entirety. No matter how much surface roughness and planarity are improved, contact is point contact in micro terms. In particular, if the package bends due to heat, ground will not be stable and good electrical characteristics (microwave characteristics) will not be obtained. Further, the heat-dissipating effect is limited, the package warps owing to evolution of heat and contact worsens, thus further destabilizing ground. Though warp preventing screws or the like below the heat-producing body can help, the only method available is to screw down the periphery of the package from above. Even if grounding is achieved, the temperature of the semiconductor may exceed the absolute maximum rating if power consumption is high when one takes thermal resistance into account.
A method of securing the package to the case (heat sink) by solder is available as a method of solving the above-mentioned problem. If the package is secured by soldering, however, it cannot be moved again. If a malfunction or the like occurs, therefore, it is necessary to replace the case in its entirety and not just the package in which the transistor, MIC or MMIC is mounted. This means that all of the circuits included in the case must be replaced. The problem that arises is an increase in cost.
Further, there is a method that uses a silicon compound as a material that may be replaced subsequently. However, since the compound is an insulator, a problem encountered is that grounding is difficult to achieve. In addition, the technique for applying the compound thinly and skillfully is difficult. This is a task requiring skill.
A method of laying an indium sheet is available as a method of achieving ground. However, the sheet is squashed by pressure from above and has no resiliency. If it is used for a long period of time, therefore, a gap may form between the package and the case. This gives rise to reliability-related problems and a change in electrical characteristics.
Further, in the case of a low-frequency circuit, sandwiching a graphite sheet between the heat-producing portion and the case has been considered. A structure of the kind illustrated in the sectional view of FIG. 20 has been proposed. Specifically, a cap filter 3 is interposed in a gap between a heat source (MPU, PA, graphic chip, etc.) 2 provided in a package 1 and the upper part of the package, a graphite sheet 4 is laid on the top surface of the package and a case (heat sink) is laid on the graphite sheet and fixed, thereby achieving mounting. If this expedient is adopted, heat is transferred in the directions indicated by the arrows. If the graphite sheet 4 is used, a non-uniform heat distribution HTD1 that prevails when the graphite sheet is not used changes to a heat distribution HTD2 owing to dissipation of heat through the entirety of the heat sink and the heat at the heat-producing portion is dissipated. FIG. 21 is a diagram for describing a DVD-RAM mounting structure. A pick-up 8 is provided below a printed board 7 on which CPUs 6a, 6b are mounted, and a PGS graphite sheet 4 is placed on a heat-producing portion such as a light-emitting diode of the pick-up 8. In the arrangement of FIG. 21, heat evolved by the CPUs and pick-up is transferred to and dissipated from an external case 9 via the graphite sheet 4.
However, it is difficult to apply the mounting structures of FIGS. 20 and 21 to mounting of a microwave circuit. The reason is that in the case of a microwave circuit, it is necessary to place a circuit board, the principle part of which is a microstrip, in a surrounding area in close proximity to the package (transistor, MIC or MMIC).
In the case of a low-frequency circuit, as shown in FIG. 21, no problems arise even if connection is by point contact. However, in the case of a package internally accommodating a microwave circuit, a surface-contact ground structure becomes necessary. In the mounting arrangements of FIGS. 20 and 21, the graphite sheet exhibits superior heat transfer in the transverse direction, as indicated by the arrows. However, since a single sheet is thin, there is little spread of heat in the transverse direction, as indicated by these drawings.
Further, the structure in FIG. 20 is such that in order to press the heat-producing portion against the graphite sheet 4, it is held down from above by the case 5. In this case, the space above the surrounding circuit also is narrowed. In the case of a microwave circuit, the space above the peripheral circuitry is reduced. As a result, even if peripheral circuitry is installed, ground above the circuit is too close and it is difficult to obtain the usual microwave characteristics. Further, in order to dissipate heat in practical terms, special hardware for strong hold-down is required.
Though the attaching method of the case is not clarified in FIG. 21, fixing the CPU package to the heat dissipating portion (the external case) by the graphite sheet 4, which is an adhesive layer, is contemplated. However, the existence of an adhesive layer enlarges thermal resistance. Further, the lower part of the package of the transistor, MIC or MMIC used in a microwave and millimeter-wave band power circuit, which evolves a large amount of heat, is greater than 100° C. Since an adhesive layer can withstand a high temperature only on the order of 100° C., the sheet constituting the adhesive layer cannot be used in mounting a microwave circuit in the manner of the mounting structure of FIG. 21. Further, since the mounting structure of FIG. 21 is for dealing with a low-frequency circuit, the special nature of microwave and millimeter-wave high-frequency circuits is not taken into consideration and there are limitations with regard to the mounting of a microwave circuit that requires the placement of a board, the principle part of which is a microstrip, in a surrounding area in close proximity to the package (transistor, MIC or MMIC).
In FIG. 22, a diagram in which pressure is applied to the package 1 is drawn in order to measure thermal resistance, and the entirety of the package 1 is subjected to pressure. This also is a structure in which the package is pressed from above. Problems the same as those of FIGS. 20 and 21 occur. That is, with a structure in which the entirety of the package of FIG. 22 is pressed down from above, the space above the surrounding circuitry takes on the same height as that of the package. In the case of a microwave circuit, the space above the circuit diminishes. As a result, even if circuitry is installed, ground above the circuit is too close and it is difficult to obtain the usual microwave and millimeter-wave characteristics. Furthermore, in a case where pressure is applied from above, there are instances where Teflon or the like having a low dielectric constant is used. However, this merely gives an assist in microwave and millimeter-wave applications and the package of a transistor, MIC or MMIC cannot be secured by pressure alone.
With regard to dissipation of heat, configurations in which the heat is brought from the heat-producing portion to the heat-dissipating portion by utilizing a graphite sheet have been considered, as seen in the Japanese Patent Application Laid-Open Nos. 8-23183, 11-110084 and Japanese Patent Application Nos. 11-149329, 10-3333202, 11-128567 and 10-51170. However, these examples of the prior art give no consideration to the problem of ground, which is specific to microwaves. Further, though the graphite sheet has a high thermal conductivity in the transverse direction and therefore has some degree of effectiveness, the sheet has little thickness and exhibits only a small amount of heat transport. For example, if the amount of heat evolved by a transistor, MIC or MMIC is greater than 100 W, it is difficult to lower the channel temperature of the transistor by more than 10° C. Further, in the case of a microwave or millimeter-wave circuit, the circuit board is very nearby and therefore it is necessary to transport heat while avoiding the circuit board. In other words, heat can only be transported from portions not related to the RF characteristic, in which case the amount of heat transported is small and satisfactory effects are not obtained.
For example, Japanese Patent Application Laid-Open No. 8-21183 discloses a cooling structure for cooling a heated member in a low-frequency circuit, i.e., a cooling structure for a heat-producing member characterized by inclusion of a heat-dissipating part made of graphite exhibiting a high degree of alignment. It is indicated that the heat-dissipating part in this cooling structure may be {circle around (1)} a heat sink for cooling an electronic part that is a member evolving heat, {circle around (2)} a sealing member for sealing an electronic part that is a member evolving heat, or {circle around (3)} a heat-dissipating member connecting an electronic part that is a member evolving heat and a heat-dissipating body for dissipating heat.
According to method {circle around (1)}, the heat-dissipating characteristic of the heat sink is excellent. However, in a case where the heat-producing body is placed inside a package, as in a microwave or millimeter-wave circuit, a problem which arises is that a satisfactory heat-dissipating effect is not obtained unless there is sufficient thermal transfer from the package to the heat sink. Further, according to method {circle around (2)}, it is required that the sheet have an adhesive function. The graphite sheet described, however, does not illustrate an adhesive function. If an adhesive is used, the thermal conductivity will decline owing to the adhesive and the heat-dissipating effect will be degraded. According to method {circle around (3)}, the connecting member has a shape of the kind shown in FIG. 1 of Japanese Patent Application Laid-Open No. 8-21183. However, owing to the small thickness of the sheet, the amount of heat transported is small. For example, if the amount of heat evolved by a transistor, MIC or MMIC is 100 W, it is difficult to lower the channel temperature of the transistor by more than 10° C.
Further, as illustrated in Japanese Patent Application Laid-Open No. 10-283650, there is an example in which a heat sink is fixed using a connecting member in a laser-beam generating apparatus. Though circuit devices can be used independently in a case where there is no relation to frequency, as in this example, the microwave circuit is such that another microwave circuit is in close proximity to the surroundings. This arrangement is not practical. Further, with the method using a connecting member, a problem which arises is that a new member referred to as a connecting member is required. That is, in a case where the package is fixed, the connecting-member material and connecting conditions are required and a problem which arises is an increase in member other than the usual screws for fastening the package.
A structure that allows heat to escape using an indium sheet is illustrated in Japanese Patent Application Laid-Open No. 5-75283. However, indium has almost no restoring force, undergoes deformation with use over an extended period of time, a gap is produced between itself and the case and the heated body may peel off. Problems arise in terms of thermal conductivity and maintenance of ground.
The foregoing is prior art for dissipating heat that has evolved from the heated body. With a high-frequency circuit or high-speed digital circuit that uses superconductivity for dealing with microwave and millimeter-wave high-frequency electromagnetic components, it is required that members be kept at extremely low temperatures. FIGS. 23A and 23B are diagrams for describing a cooling structure. FIG. 23A is a perspective view in which an upper cover 10 of a high-frequency circuit device employing superconductivity has been removed, and FIG. 23B is a sectional view taken along line A′A of the upper cover. A base metal board (Invar, Kovar, copper) 13 on which a superconducting RF circuit board 12 has been mounted is secured to a case 11 by board-mounting screws 14a to 14f. Indium sheets 15a, 15b for removing heat are laid between the case 11 and base metal board 13 and between the superconducting RF circuit board 12 and the base metal board 13. Furthermore, the case 11 is secured on a cold head 16 by screws 17a, 17b via an indium sheet 15c. The arrangement is such that a coolant such as LN or LHe is passed through the interior of the cooling end 16 or such that cooled He gas fed from a refigerator (not shown) is passed through the cooling end. At an operating temperature of about 4K that adopts liquefied He (LHe) as the coolant, a superconducting RF circuit that uses an Nb superconducting film or YBCO, BSCCO film can be cooled. At an operating temperature of about 77K that adopts liquefied nitrogen as the coolant, or at 50 to 80K using a refigerator, a superconducting RF circuit that uses YBCO or BSCCO film can be cooled. In FIG. 23A, reference numeral 18 denotes a superconducting film pattern on the superconducting RF circuit board 12, reference numerals 19a, 19b denote coaxial connectors, and reference numerals 20 to 20d denote holes for receiving screws.
The evolution of heat by a circuit in a portion to be cooled is smaller by an order of magnitude in comparison with ordinary semiconductor circuits and wiring. However, heat that penetrates from the exterior of the cooling device penetrates the portion to be cooled (the superconducting RF circuit board 12) through the case 11. The rise in temperature owing to penetration of heat is lowered by the cold head 16. However, the indium sheets 15a to 15c exhibit almost no restoring force and have adhesion to with respect to many case materials and boards. When the case 11 is removed, therefore, the indium sheet 15c undergoes shape deformation and re-use in the same shape is difficult. In case of re-use, a gap tends to form between the sheet and case, temperature unevenness develops within the board and often it is difficult to achieve active operation and transmission-circuit operation by superconductivity with a high degree of reproducibility.
Further, in the cooling structure of FIGS. 23A, 23B, problems are EMC, vibration in the internal circuitry and degradation of characteristics. The case 11 of the microwave or millimeter-wave circuit usually has the cover 10 and a bottom cover fixed to it by screws 17a to 17b. Since a gap is formed in such case, radio waves leak from the gap, thereby bringing about the problems EMC, vibration in the internal circuitry and degradation of characteristics. EMC involves two phenomena, namely leakage of radio waves to the outside and penetration of external radio waves. Vibration is caused by part of the output being fed back to the input through the gap. Further, the degradation of characteristics also is caused by coupling between circuits through the gap. For example, this leads to a frequency characteristic exhibiting a break.